Dichlorvos has been used by the fish farming industry to control sea lice for many years. The Department of the Environment has estimated that 15 tonnes of the chemical were released in UK waters in 1990.
Dichlorvos is highly toxic to many marine animal larvae at low concentrations and has been placed on the official "red list" for that reason. The Government is committed to reducing inputs of substances on the list to marine waters to 50% of 1985 values by 1995. Dichlorvos has a temporary licence which expires this year.
In April, Wester Ross Salmon took representatives of the Scottish Office and the Highland River Purification Board to the Faroe Islands to see the new treatment in use. The company is expecting to conduct further trials in conjunction with peroxide manufacturer Interox in the next few weeks.
Fish are exposed to concentrations of about 2ppm of peroxide, which causes bubbles of oxygen to form inside the lice and kills them by physical rather than chemical means. Hydrogen peroxide is widely used in water treatment and it is expected to have minimal impact on the marine environment.
The chemical costs of hydrogen peroxide dosing are likely to be about 1,000 times higher than dichlorvos treatment, but Wester Ross Salmon is optimistic that the advantages will make it cost-effective. The company believes that peroxide will reduce stress and damage to the fish and overcome the growing problem of louse resistance to dichlorvos.
Should peroxide prove effective, regulatory pressure is likely to work in its favour. Environmental quality standards proposed last November by the Department of the Environment (ENDS Report 202, pp 29-30) will probably prove sufficiently tight to restrict the use of dichlorvos in many sea lochs where water circulation is poor.
Although optimistic about the new treatment, Wester Ross Salmon is cautious in case the Government's Veterinary Products Committee decides that peroxide requires a licence. This would mean a delay of several years before the treatment could be used and would restrict its availability to prescription only.